In the automatic testing of electrical circuits, test probes of various configurations are used, depending upon such factors as the type of electrical device under test, the spacing between test points, and the like. The present invention is applicable to a type of contact test probe which is rotated as it moves into contact with a test point or node on the unit under test.
In order to appreciate the improvements provided by this invention, one type of prior art test probe will first be described. This test probe assembly comprises a conventional spring loaded contact probe generally including an outer receptacle, a removable plunger or probe member, and a barrel in the receptacle. In such devices, the plunger or probe member is mounted in the barrel and extends outwardly from it. The barrel includes a spring so that the plunger or probe member is supported in the barrel by spring pressure. The probe member can be biased outwardly a selected distance by the spring and may be biased or depressed inwardly into the barrel a selected distance under force directed against the spring. The probe member generally has a probing end or tip which makes contact with selected test points on the electrical device under test. A printed circuit board would be an example of a unit under test.
There are many applications in which the probing end of the test probe must grip onto a particular test point or node during probing. Therefore, the test probe must be designed so that it stays in good contact with the circuit test point being probed. In order to ensure good contact during probing, prior art test probes have included a large number of different probe tip configurations designed to grip onto the test node to make good contact when spring pressure is applied to the probe during testing.
Examples of instances in which good contact is required between the test probe and the circuit test node include where the board under test has been in a dirty environment, or where no de-fluxing was conducted after soldering; and therefore, the test probe must grip or bite into the conductive surface of the test node to ensure circuit continuity.
A conventional way to ensure good probe contact is by use of a "helical" test probe assembly which includes a cylindrical barrel having an axial passage and a movable test probe which rotates as it slides axially in the passage. The barrel has a coil spring inside it which applies a spring bias to a plunger during use. The plunger can have a small lateral post extending outwardly and engaged in a spiral groove extending lengthwise along the barrel. The post engaged in the groove provides a means for controlling movement of the probe relative to the barrel during use. When the plunger portion of the probe assembly is depressed against the spring pressure, the post rides upwardly in the spiral groove and causes the probe shaft to rotate about its axis during probing. By twisting about its axis, the bottom contact tip portion of the probe can bite into the contact point on the unit under test to provide better contact during testing.
An improved helix probe is described in U.S. patent application Ser. No. 07/431,477, filed Nov. 3, 1989, and entitled "Electrical Test Probe Having Rotational Control of the Probe Shaft." This application is owned lo by the assignee of this application and is incorporated herein by the reference.
There are certain drawbacks to using a helical probe with a conventional probe tip configuration. Problems can arise when probing test points which project from the board surface nonuniformly. An example is a solder bump in which an exposed lead from a chip on the opposite side of the board projects through the solder bump at an angle. If this test point is being probed by a helix probe with a conventional contact tip, such as a serrated head or a funnel or cone shaped head, the tip can be get stuck as it turns, which can break off the lead, cause the probe to bind, or break off the head of the contact tip. Contact with the angled lead tends to pull the head of the probe over to one side and can bind the probe in a necked-down lower portion of the helix probe. There tends to be very little room for lateral motion of the probe relative to the barrel in this portion of the probe, whereas a certain amount of play would otherwise resist binding.
The present invention provides a contact tip for a test probe of the helix probe variety in Which contact can be made with an irregular test node while the probe is being rotated, without causing binding of the probe or any other undesired resistance to the normal rotation of the probe.